Fire alarm systems and mass notification systems typically use distributed notification devices to notify the public of the presence of fire, smoke, and other conditions. In these systems, a notification appliance circuit (NAC) is often used to connect the notification devices to a control panel.
Power for the notification device is provided over the NAC from the control panel. Primary power to control panel may be, for example, AC power derived from a utility grid. Many systems also include a battery backup power supply at the control panel in order to maintain operations when the main power supply is faulty or interrupted.
Power supplied through the NAC to notification devices may be limited by the worst case voltage to the NAC and by the voltage drop across the NAC wiring. This may result in less than optimal coverage for NAC circuits.
For example, an NAC may be designed to have 30 notification devices, each drawing 100 milliamps and having a rated spacing of 10 feet at a working voltage and current. Thus, the NAC would provide notification coverage of 300 feet. Under real world conditions, because the voltage drops from the supply through various system components, for example, a panel, circuit wiring, and the wiring of the NAC itself, the NAC may be limited to fewer devices and less coverage length because the working voltage and current for all the devices may not be provided over the entire NAC as originally designed.
One commonly used type of NAC system makes use of reverse polarity circuits that are supervised by an end of the line resistor. The notification devices themselves may be simple on/off devices with a diode that maintains the notification devices in an off state when the power on the NAC has a first polarity. The diode completes the power circuit for the notification device when the circuit polarity is reversed from the first polarity to a second polarity. Each of the notification devices has the same or similar operating characteristics in this type of system.
The NAC circuit has a supervisory state, in which the polarity of the voltage on the NAC circuit wires is such that the diodes within the notification devices are reversed biased. In the supervisory state, the NAC circuit is supervised, but the notification devices are not active.
When the polarity of the voltage on the NAC circuit wires is reversed, the NAC circuit is in an active state. The diodes within the notification devices are forward biased, allowing current to flow through the notification devices to activate the notification devices.
A notification device may provide both visual as well as audible signaling. The visual signaling can be produced by a strobe circuit that includes a light source, such as a gas filled flash tube or light emitting diodes (LEDs), as well as a driver or trigger circuit that provides the necessary voltage and current to either the light source. The strobe circuit is typically powered by a storage capacitor, which must be recharged with current from the NAC circuit after each flash produced by the strobe circuit. The current required to recharge the capacitor after every flash represents a significant portion of the total current requirement of each notification device.